1. Field of the Invention
The present invention relates to an amplifying apparatus (which will also be referred to as “amplifier” hereafter) with an automatic level controller (which will be referred to as “ALC” hereafter) for controlling the output level of an amplifier by controlling the gain of a variable gain amplifier included therein.
2. Description of the Related Art
Conventional automatic level controllers are well known, having a function for controlling the gain of a variable gain amplifier so as to output a constant-level output signal. These automatic level controllers work as follows. That is to say, the output signal level is compared with a reference level. In a case that the output signal level is higher than the reference level, the automatic level controller reduces the gain thereof. On the other hand, in a case that the output signal level is lower than the reference level, the automatic level controller increases the gain thereof. Such increase and reduction are performed at predetermined time intervals (frame period), thereby enabling stable gain control (see Patent document 1).
[Patent Document 1]
Japanese Patent Application Laid-open No. 2003-173211
With conventional automatic level controllers, the gain of an amplifier is adjusted in response to fluctuation of the output-signal level around a reference level. Data communication circuits and so forth operate without any problems as long as the signal level is maintained, regardless of how frequent the amplifier gain is adjusted. Accordingly, with the conventional automatic level controllers, the frame period is designed so as to prevent error due to noise or the like. Thus, the frame period is preferably set to as short a period as possible.
However, an amplifier with an ALC for automatic control of the output signal level of audio signals so as to be within a predetermined level has different problems. That is to say, with an arrangement in which the gain is controlled exactly in response to fluctuation of the output signal level around the reference level, the output signal level changes exactly in response to the change in gain, leading to a problem of unnatural-sounding audio signals from the perspective of auditory sensation.
Furthermore, an arrangement in which action for increasing the amplification gain (i.e., recovery action) and action for reducing the amplification gain (e.g., attack action) are performed at the same intervals (frame period) also has a problem of output of unnatural-sounding audio signals from the perspective of auditory sensation.
For example, let us consider a case of the attack action. In this case, with an arrangement in which gain control is performed at excessively long intervals, an audio signal input suddenly after a period of no audio signals is amplified with an excessively high gain, leading to a problem of an excessively high output-signal level. Furthermore, this leads to distortion of the output-signal waveform, and such distortion remains for a while. Thus, in a case of the attack action, the gain control is preferably performed with short intervals from the perspective of auditory sensation.
Next, let us consider a case of the recovery action. In this case, with an arrangement in which gain control is performed at excessively short intervals, gain is increased immediately in response to a stop of audio signals. This leads to amplification of noise, resulting in a problem of audio signals with extremely poor quality from the perspective of auditory sensation. Furthermore, in a case of the recovery action, with an arrangement in which gain control is performed at excessively long intervals, a small audio signal suddenly input after a period of large signals leads to a problem that such a small audio signal is amplifies with an excessively low amplification gain for a while, resulting in a situation in which such a small audio signal cannot be heard for a while. Thus, control of the recovery action is preferably performed with suitably-long intervals from the perspective of natural-sounding auditory sensation.